Electrical breakdown strength of interfaces between solid insulation and transformer oil with variable water content

Pressure equalized cable terminals and high voltage connectors are bottlenecks in a subsea distribution system for electric power. Connectors known as wet mateable connectors must be able to operate in sea water at high hydrostatic pressure. The use of high viscosity liquid like grease, gel or oil being suggested in these terminations introduces a parallel liquid-solid interface. Assessment of the electrical strength of such interfaces is therefore important for proper design of Subsea accessories. This paper describes breakdown mechanisms due to absorption of water. Emphasis will be put on the experimental examination of the AC 50 Hz breakdown strength of interfaces between the XLPE insulation and transformer oil with variable water content. An experimental test setup has been designed to examine factors that influence the interface´s electrical strength according to IEC156 and ASTM D1816 standards. The effect of water saturation ratio on a liquid-solid dielectric interface in the temperature range of 20-90degC is presented.